Solid state relay

ABSTRACT

A solid state relay prevents the damage to photothyristor couplers when their performance characteristics vary slightly and enables phase angle control of an AC power supply using a less costly configuration than relays with photothyristor couplers having zero-crossing functions. The solid state relay includes a pair of input terminals configured to receive a control signal, a pair of output terminals configured to be connected to an AC power supply, a thyristor in parallel to the pair of output terminals configured to switch the AC power supply in response to the control signal. The solid state relay further includes two photothyristor couplers, each having a photothyristor, that are connected in series at a midpoint connection. The two photothyristor couplers are connected in parallel to the thyristor and are configured to control the thyristor. The solid state relay further includes two surge arresters connected in series at a midpoint connection that is connected to the photothyristor midpoint connection. The two surge arresters are connected in parallel to the pair of output terminals and are configured to block excessive voltage levels.

BACKGROUND OF THE INVENTION

The invention is directed to a solid state electronic relay.

An example of an existing solid state relay is shown in FIG. 3. This solid state relay has two input terminals, IN₁ and IN₂, and two output terminals, OUT₁, and OUT₂. Between the input and output terminals are a thyristor THY, two photothyristor couplers, PHT₁ and PHT₂, a snubber circuit K and a surge arrester SA.

In operation, an input signal is applied to input terminals IN₁ and IN₂ to control the AC output. AC circuit Z, consisting of AC power supply and load L, is connected to output terminals OUT₁ and OUT₂. In this example, the input signal applied to input terminals IN₁, and IN₂ is DC, however, it is also possible to use AC input. The photothyristor couplers PHT₁ and PHT₂ drive thyristor THY in response to the input signal applied to input terminals IN₁ and IN₂. In turn, the thyristor THY controls the current flow in AC circuit Z.

Photothyristor couplers PHT₁ and PHT₂ can both can be turned ON only when the voltage from power supply AC approaches the zero voltage cross-point. The photothyristor couplers have what is known as a zero-crossing function. Each of the photothyristor couplers PHT₁ and PHT₂ has a built-in zero-crossing detector circuit (not shown), a light emitting diode D₁, or D₂ as a luminous element, and a photothyristor TR₁, or TR₂. Light emitting diodes D₁ and D₂ are connected in series to input terminals IN₁ and IN₂ ; photothyristors TR₁ and TR₂ are connected separately in series to output terminals OUT₁ and OUT₂.

Snubber circuit K prevents the accidental operation of thyristor THY which might be induced by back current flow when load L is inductive. In this example, the snubber circuit has two resistors, R₁ and R₂, and two capacitors, C₁ and C₂ connected in series.

A surge arrester SA, which in this example is a varistor, protects thyristor THY and photothyristors TR₁ and TR₂ in case an overvoltage is generated by AC circuit Z.

R₀ is a resistor which sets the gate current for thyristor THY.

When no DC input signal is applied to input terminals IN₁ and IN₂ the light emitting diodes D₁ and D₂ do not emit light. Thus, photothyristors TR₁ and TR₂ are not conductive, and thyristor THY is off. As such, power is not supplied from power supply AC to load L.

In this state, the voltage from power supply AC is divided between the two photothyristors TR₁ and TR₂ which are in series. For example, if the voltage of power supply AC is 400 V rms, a voltage of 200 V rms (peak voltage of 282 V) is applied to each of photothyristors TR₁ and TR₂.

When a DC input signal is applied to input terminals IN₁ and IN₂, light emitting diodes D₁ and D₂ in photothyristor couplers PHT₁ and PHT₂ emit light. Referring to FIG. 4, if the input signal is applied at a point in time when the waveform is not within the region delta V (FIG. 4(b)) of low voltage near the zero-crossing point of the power supply voltage (at time t₁ in FIG. 4(b), for example), photothyristors TR₁ and TR₂ do not turn ON, so thyristor THY does not go ON and power is not supplied from power supply AC to load L.

When the input signal is applied at a point in time when the waveform is within the region delta V of low voltage near the zero-crossing point of the power supply voltage (at time t₂ for example), photothyristors TR₁ and TR₂ will turn ON. Thyristor THY will then turn ON and the power is supplied from power supply AC to load L.

Photothyristors TR₁ and TR₂ only turn ON when the power supply voltage is in the low-voltage region delta V near the zero crossing point. Thus, a high voltage is never applied to photothyristors TR₁ and TR₂.

When the input signal applied to input terminals IN₁ and IN₂ is turned OFF and the power supply voltage falls below the holding current of photothyristors TR₁ and TR₂ (as at time t₃), elements TR₁ and TR₂ are no longer conductive. Thyristor THY is also no longer conductive, and so the flow of electricity from power supply to load L is cut off.

The conventional solid state relay described above employs photothyristor couplers PHT₁ and PHT₂ that have zero-crossing functions. Because elements TR₁ and TR₂ vary somewhat in manufacture, their turn on time is necessarily slightly out of phase. However, because of the zero-crossing function, there is no possibility that elements TR₁ and TR₂ will be damaged by excessive power from power supply AC.

Photothyristors TR₁ and TR₂ are protected by the fact that when they are not conductive, the power supply voltage is divided between them, and when they are conductive, the power supply voltage is always near the zero crossing point. The maximum value of the power supply voltage is, therefore, never applied to either photothyristor TR₁ or TR₂.

However, in a conventional relay, such as the one shown in FIG. 3, the phase angle at which the AC power supply switches ON and OFF cannot be controlled, due to the zerocrossing function.

This poses a problem, since there are a number of devices in which it is necessary to control the phase angle at which AC power is applied, such as motor controllers or dimmers. As discussed above, in a relay that employs photothyristor couplers PHT₁ and PHT₂ with zero-crossing functions (FIG. 3), when an input signal is applied to input terminals IN₁ and IN₂ the output terminals OUT₁ and OUT₂ do not immediately conduct, but rather conduct depending on the position of the supply voltage waveform.

It would be conceivable to address this problem by substituting elements without a zero-crossing function for photothyristor couplers PHT₁ and PHT₂ in the solid state relay shown in FIG. 3. If this were done, output terminals OUT₁ and OUT₂ would immediately conduct when an input signal is applied to input terminals IN₁ and IN₂, regardless of the phase angle of the AC waveform. In this way the phase of switching could be controlled.

However, if photothyristor couplers PHT ₁ and PHT₂ are used which completely lack a zero-crossing function, the following problem arises. Assuming that the voltages at both terminals of photothyristors TR₁ and TR₂ are V₁ and V₂ respectively, and that photothyristors TR₁ and TR₂ are both OFF, as shown in FIG. 5 (the period labeled T_(a)) the voltage from the AC power supply is divided between photothyristors TR₁ and TR₂, and so is not applied in full to either of the photothyristors. Even if the supply voltage is at its maximum value V_(max), the terminal voltage on either photothyristor will be V₁ =V₂ =V_(max) /2.

One of photothyristor couplers PHT₁, and PHT₂ will, in general, have a time lag, delta T, in its turn on time due to variations in its performance characteristics caused by the manufacturing process, as mentioned above. If thyristor TR₁ turns ON slightly ahead of photothyristor TR₂ and the supply voltage is in the vicinity of its maximum value, then the maximum voltage V_(max) will be applied momentarily to photothyristor TR₂ alone. The voltage across the photothyristor TR₂, V₂, will then go to V_(max), and element TR₂ will be damaged.

To prevent photothyristors TR₁ and TR₂ from being damaged in this way, elements with breakdown voltages sufficiently higher than the maximum value V_(max) of the power supply voltage are needed.

For example, if the voltage of the AC power supply is 400 V rms, zero-crossing function photothyristor couplers PHT₁ and PHT₂ each with a breakdown voltage of 600 V could be used. If photothyristor couplers PHT₁ and PHT₂ without a zero-crossing function are used, however, they would each need to have a breakdown voltage of 1200 V, which would drive up the cost of the relay.

SUMMARY OF THE INVENTION

The invention provides a solid state relay (SSR) with a protective circuit to protect photothyristors from damage when high voltage is applied. The invention is especially applicable for relays that have photothyristors that lack a zero-crossing function, which, in general, are less expensive than those having the zero-crossing function.

The invention prevents photothyristors used in a solid state relay from being damaged when the photothyristors have different performance characteristics. It also makes it possible to provide phase-controlled switching using an economical construction that employs photothyristor couplers without zero-crossing functions.

The invention includes a solid state relay having: input terminals to which a signal is applied to control the output switching of an AC power supply; thyristors that switch the output of the AC power supply and are connected between the input terminals and between the output terminals connected to the aforesaid AC power supply; and two photothyristor couplers, which drive the thyristor in response to the aforesaid input signal. The luminous elements and photothyristors which comprise the two photothyristor couplers are connected, respectively, in series on the input terminal side and the output terminal side.

One of the distinguishing features of this invention is that a series combination of two surge arresters is connected in parallel to the output terminals and a short circuit is provided between the midpoint connection of the two surge arresters and the midpoint connection of the two input photothyristors.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a circuit diagram of a solid state relay.

FIG. 2 is a waveform diagram illustrating the terminal voltages applied to the photothyristors in the solid state relay.

FIG. 3 is a circuit diagram of a conventional solid state relay.

FIG. 4 is a waveform diagram showing the relationship between the input signal and the voltage between the output terminals in the solid state relay of FIG. 3.

FIG. 5 is a waveform diagram showing the relationship between the voltage between the output terminals in the solid state relay of FIG. 3 and the terminal voltages applied to the two photothyristors.

DESCRIPTION

FIG. 1 shows a circuit diagram of a solid state relay, in which Z is the AC circuit; L is the load; AC is the AC power supply; IN₁ and IN₂ are input terminals to which an input signal is applied to control the current flow in the AC output circuit; OUT₁ and OUT₂ are the output terminals connected to AC circuit Z; THY is a thyristor which switches AC circuit z; K is a snubber circuit to prevent the accidental operation of thyristor THY; R₁, R₂, C₁ and C₂ are the resistors and capacitors comprising snubber circuit K; and R₀ is a resistor to set the gate current to thyristor THY.

In this embodiment, two photothyristors PHT₁ and PHT₂ drive thyristor THY according to an input signal. In contrast to conventional relays, photothyristors PHT₁ and PHT₂ do not have a zero-crossing function. Thus, they can turn ON immediately regardless of the phase angle of the voltage waveform from the AC power supply. The light emitting diodes D₁ and D₂ which constitute photothyristor couplers PHT₁ and PHT₂ are connected in series with input terminals IN₁ and IN₂ ; photothyristors TR₁ and TR₂ are connected in series with output terminals OUT₁ and OUT₂.

Also in this embodiment, two surge arresters SA₁ and SA₂ connected in series with one another are connected in parallel with output elements OUT₁ and OUT₂. Resistors R₁ and R₂ connected in series with one another, act as a voltage divider and are connected in parallel with photothyristors TR₁ and TR₂. A short circuit is provided from the midpoint a between the resistors and the two photothyristors TR₁ and TR₂ to the midpoint b between the two surge arresters SA₁ and SA₂.

In this example, surge arresters SA₁ and SA₂ are varistors, although arrester tubes could also be used. The series circuit of surge arresters SA₁ and SA₂ serves to protect thyristor THY and photothyristors TR₁ and TR₂ from overvoltage generated by any back current in circuit Z. Surge arrester SA₂ prevents photothyristors TR₁ and TR₂ from being damaged as a result of any differential in their Turn on time.

Resistors R₁ and R₂ supplement the action of surge arresters SA₁ and SA₂. Since the two resistors having the midpoint c divide the voltage, excessive voltage will not be applied to either of photothyristors TR₁ and TR₂ even if there is a differential in their Turn on time. In this example, photothyristor couplers PHT₁ and PHT₂ are identical, so resistors R₁ and R₂ have the same resistance value.

The photothyristors PHT₁ and PHT₂ used in the relay shown in FIG. 1 are the type which do not have a zero-crossing function. Thus, they can turn ON immediately in response to an input signal and drive thyristor THY, regardless of the phase angle of the voltage waveform from the AC power supply.

There may be a differential, delta T, in the turn on time of photothyristor couplers PHT₁ and PHT₂ due to a variation in their performance characteristics caused by the manufacturing process. This differential leads to variations in the voltage applied to photothyristor couplers PHT₁ and PHT₂, as shown in FIG. 2, however, the invention limits such variations, as described below.

For example, if photothyristors TR₁ and TR₂ are both off (time period T_(a)) the supply voltage from the AC power supply is divided between TR₁ and TR₂ (V₁ and V₂). The full voltage is not applied to either of the photothyristors TR₁ and TR₂. Even when the power supply voltage is at its maximum value V_(max), the terminal voltages will still be V₁ =V₂ =V_(max) /2.

If an input signal is applied to input terminals IN₁ and IN₂, photothyristor TR₁ may turn ON before photothyristor TR₂ by a time period of delta T. If the voltage from the AC power supply is in the vicinity of its maximum value V_(max), the maximum voltage V_(max) is divided by resistors R₁ and R₂ and a lower voltage will be actually applied to photothyristor TR₂.

In addition, if a high voltage threatens to be applied to a photothyristor, such as TR₂, due to a differential in turn on time, the resistance value of surge arrester SA₂ decreases, and the current flows through the photothyristor that turns ON first, TR₁ , through surge arrester SA₂. As a result, the voltage applied to the terminals of photothyristor TR₂ is voltage V', which is sufficiently lower than the maximum voltage V_(max) of the AC power supply, as shown in FIG. 2. If photothyristor TR₂ were to turn ON before photothyristor TR₁ by a period of delta T, the same situation would apply.

As explained above, the invention allows photothyristor couplers that do not have zero-crossing functions and have lower breakdown voltages to be employed. The required breakdown voltage is approximately the same as that required for photothyristor couplers with zero-crossing functions used in conventional relays, such as that shown in FIG. 3.

For example, if the voltage of the AC power supply is 400 V rms, and photothyristors PHT₁, and PHT₂ without a zero-crossing function are used without the short circuit, components must be used which have a breakdown voltage of 1200 V, as was discussed above. On the contrary, the circuit in the embodiment shown in FIG. 1 uses photothyristors PHT₁ and PHT₂ without a zero crossing function, yet it can get by using components with a breakdown voltage of 600 V.

In addition to above, even if surge arresters SA₁ and SA₂ for the 200 V system which is half of 400 V, as a set they constitute a 400 V system, which can adequately protect thyristor THY.

In the embodiment pictured in FIG. 1, photothyristor couplers PHT₁ and PHT₂ do not have a zero crossing function. However, this invention could obviously also be applied if these components had a zero crossing function. Also, although in this embodiment the input signal applied to terminals IN₁ and IN₂ is a DC signal, the invention would also apply if an AC signal were used.

In the solid state relay of this invention, if there is a differential in the turn on time of the two photothyristor couplers due to variations in their characteristics occurring during manufacture, overvoltage applied to one of the photothyristors will be absorbed by a surge arrester to effectively prevent the photothyristor from being damaged.

Thus phase angle control can be realized using cheaper photothyristor couplers with no zero crossing function. 

What is claimed is:
 1. A solid state relay, comprising:a pair of input terminals configured to receive a control signal; a pair of output terminals configured to be connected to an AC circuit, the AC circuit comprising a load and an AC power supply; a thyristor in parallel to the pair of output terminals and configured to switch the AC power supply in response to the control signal; two photothyristor couplers connected in series at a first midpoint connection, wherein the two photothyristor couplers are connected in parallel to the thyristor and are configured to control the thyristor, each photothyristor coupler comprising a photothyristor; and two surge arresters connected in series at a second midpoint connection, wherein the two surge arresters are connected in parallel to the pair of output terminals, the two surge arresters being configured to protect the thyristor and the two photothyristor couplers from an excessive voltage level generated by a back current from the AC circuit, wherein the first midpoint connection is connected to the second midpoint connection.
 2. A solid state relay according to claim 1, wherein the two photothyristor couplers are configured to be operable at all possible values of AC voltage phase.
 3. A solid state relay according to claim 1, wherein the two photothyristor couplers comprise zero-crossing detection circuits.
 4. A solid state relay according to claim 1, wherein the control signal received at the pair of input terminals controls a phase angle of the AC power supply.
 5. A solid state relay according to claim 1, wherein the photothyristor coupler comprises a photothyristor connected to the thyristor and a light emitting diode connected to the pair of input terminals.
 6. A solid state relay according to claim 1, wherein the surge arrester is a varistor.
 7. A solid state relay according to claim 1, wherein the surge arrester is an arrester tube.
 8. A solid state relay according to claim 1, further comprising a resistor connected to the photothyristor configured to divide a voltage load generated by the AC power supply.
 9. A solid state relay according to claim 1, wherein the thyristor comprises a three terminal device.
 10. A solid state relay according to claim 1, further comprising two resistors connected in series at a third midpoint connection, the two resistors being connected in parallel to the thyristor and being configured to divide an AC voltage generated by the AC power supply, wherein the third midpoint connection is connected to the first midpoint connection. 